Conventional safety systems for machines having power stroke operation such as presses, riveters, and the like, often employ a safety circuit which must be enabled by the proper positioning of a sensor bar at the work area free of obstructions before a start switch can be effective to actuate a power stroke of the machine. In one such system the start switches enable a solenoid to drop the probe toward the work area. If there is no obstruction, as the probe arrives at the work area it trips a switch which powers the ram. In another system, closing the start switch enables a solenoid to permit the probe to drop to the work area, whereupon it trips a switch and fires the power ram. While such arrangements address the simple problem of preventing firing of the power stroke machine, they do not address the more complex problems introduced by misuse and abuse of the safety system. For example, once the first power stroke occurs subsequent strokes can often be initiated by holding down the start switch and simply raising and lowering the probe so that it becomes the operating switch. The safety factor introduced by the use of two start switches in series is overcome by taping down or otherwise holding down one or both of them. In another circumvention the start switch is quickly released and re-closed to initiate another power stroke while the probe remains down at the work area continually without being retracted for insertion of each new workpiece. These avoidances of the proper operation of the safety system create serious hazards for the operator. Often the operator may properly be lifting the probe to insert a new workpiece while inadvertantly keeping the start switch closed. This, too, can result in a dangerous situation: in riveting machines, rivets which strike the hardened work surface of the machine in the absence of a workpiece can become lethal, high-speed projectiles hazardous to all in the area.